To solve problems of the O/W emulsion drilling fluid applied in deep formation， an O/W emulsion drilling fluid with high-temperature tolerance was developed by using self-made zwitterionic emulsifier SKT-1 with high-temperature tolerance， auxiliary emulsifier Span-80， tackifier Dristemp， filtrate loss reducer sulfomethal phenolaldehy resin（SMP-I）and amphoteric ion coating agent FA-367， and so on. The stability， temperature-resistant property， rheological property at high temperature and high pressure and contamination-resistant property of this drilling fluid were studied. The results showed that the optimum formula of the drilling fluid was obtained as follows：2% sodium bentonite+2.5% SMP-I+0.2% Dristemp+0.2% FA-367+0.2% NaOH+ 2% emulsifiers（SKT-1∶Span-80=3∶1）+0.5% aluminium stearate， with 5∶5 volume ratio of diesel oil and water. The drilling fluid system had good rheological property， which density was adjusted between 0.90 to 0.98 g/cm³ and was stable at 210℃. Meanwhile， this drilling fluid had the ability to resist 20% water， 20% oil， 2% NaCl and 15% cuttings showing little damage to reservoir， which met the drilling requirements of deep formation.